1. Technical Field
The present disclosure relates to an electronic device having a flexible cable and contrived so that electromagnetic noise derived from the flexible cables is reduced.
2. Description of Related Art
In recent years, digital interfaces serving for interconnecting one electronic device to another, or provided within electronic devices have been advanced toward increasingly higher speeds. For example, high-speed serial interfaces using differential signals, such as LVDS (Low Voltage Differential Signaling), HDMI (High-Definition Multimedia Interface) and USB (Universal Serial Bus), are used in AV (Audio-Video) or other electronic devices. As digital signals transmitted in digital interfaces (communication cables) become higher and higher in speed, electromagnetic noise radiated from the communication cables increases more and more. Therefore, in view of suppression of the electromagnetic noise from electronic devices, it is quite important to suppress radiation of electromagnetic noise from communication cables serving for interconnecting one electronic device to another, or from communication cables provided within electronic devices.
In many electronic devices, flexible cables such as flexible flat cables are used as communication cables for interconnecting one component to another, e.g. one circuit board to another, within electronic devices. A flexible flat cable as an example has a structure that a plurality of signal lines formed of plate-like conductors placed in parallel with a specified distance therebetween are sandwiched by two resin films.
In order to suppress radiation of electromagnetic noise from communication cables in high-speed digital interfaces, there is a need for satisfying matchings between respective characteristic impedances of on-circuit-board lines, connectors, communication cables or other transmission path elements and element-to-element characteristic impedances among the transmission path elements. For example, it is important to satisfy a matching between characteristic impedances of flexible flat cables used as communication cables.
The characteristic impedance of a flexible flat cable depends on thickness and width of a conductor constituting signal lines, distances between the signal lines, e.g. chassis or other conductors located in vicinities of the cable. Therefore, in a case where, for example, a flexible flat cable is partly floated in the air while its rest is fixed to the chassis, the characteristic impedance of the flexible flat cable breaks down, and it becomes hard to take a matching with a characteristic impedance of a connection-destined transmission path element.
As a method for suppressing noise radiation from a flexible flat cable, a method of providing a metal shield layer on both surfaces of the flexible flat cable over its entire length is disclosed in Japanese Patent Application No. 2009-181792 A. Providing the metal shield layer makes it possible to obtain a stabilization of the characteristic impedance and a shielding effect for electromagnetic noise.
Japanese Patent Application No. 2010-122550 A discloses a structure that a flexible flat cable connected to a circuit board and extending between a chassis and another circuit board is fixed to the chassis via a bonding member of a specified thickness. Keeping distances from the chassis and the another circuit board at specified values reduces effects of electromagnetic noise from the chassis and the another circuit board to the flexible flat cable.